1. Field of the Invention
The invention relates to the field of hearing protectors, especially to an earmuff hearing protector including an electrical circuit with noise limiting features.
2. Description of the Prior Art
It has long been known that a person's ears are subject to permanent damage from high volume or high intensity noises. In the past, to reduce the potential for hearing loss, various forms of ear protectors such as earmuffs or ear plugs have been utilized. One common problem with such noise suppressors was that the wearer could not hear normal level sounds such as conversation, alarms, or other aural signals.
To solve this problem, earmuffs or noise limiters were provided with means for receiving and amplifying non-harmful sounds which nonetheless provided some dampening of harmful, more intense noise. U.S. Pat. No. 3,306,991 disclosed a system having a microphone, a pre-amplifier, a limiting amplifier and a speaker enclosed in a set of earmuffs such that the user could hear normal level sounds but sounds over a certain level would be limited. The patent disclosed the concept of limiting sound levels but did not show any particular electrical circuit configurations to perform this function beyond mentioning that an automatic gain control system could be used.
U.S. Pat. No. 3,394,226 disclosed a hearing protector which included an input audio filter to remove undesirable audio frequencies from the sounds received by a microphone thereby removing potentially harmful portions of the audio spectrum.
U.S. Pat. No. 3,683,130 disclosed a system which included a series of filter networks to shape the frequency response of the amplifier and reverse-biased clipping diodes to clip signals above a desired amplitude. Another diode clipping circuit was disclosed in U.S. Pat. No. 4,224,470.
U.S. Pat. No. 4,064,362 disclosed an automatic gain control system to limit the output signal level from the microphone and thereby the signal level transmitted by the speaker contained in the headphone system.
It is desirable that the electrical circuitry performing the frequency filtering and noise limiting functions be simple. The simplicity reduces the cost and the size of the unit, allowing the earmuff to be used more widely in cost sensitive and space sensitive environments. All of the prior art devices used separate circuit elements to perform the limiting function, thereby increasing their size and cost.